826 



ELECTRICITY. 



It In the course of its revolution. By turning the 

 button or the index, the needle may be brought into 

 this or any other required position with regard to the 

 ball. It is found by experiment, that the angle of 

 torsion of the silk fibre is, within a certain range of 

 distance, very nearly in the direct ratio of the force 

 which acts in producing the torsion; and, therefore, 

 if the two balls be placed in contact by turning the 

 button, and then similarly electrified, the distance to 

 which they are repelled by the angular motion of the 

 suspended ball affords a measure of the repulsive force 

 exerted. In like manner, the distance which the sus- 

 pended ball is made to move, when it is attracted by 

 the fixed ball, when the two liave opposite electrici- 

 ties, gives accurate measures of the attractive forces. 

 V. It had long been observed, that the quantity of 

 electricity which bodies are capable of receiving, 

 does not follow the proportion of their bulk, but 

 depends chiefly upon the extent of their surface. It 

 was found, for example, that a metallic conductor, 

 in the form of a globe or cylinder, contains just as 

 much electricity when hollow, as it does when solid; 

 from which it was inferred, that electricity does not 

 extend throughout the mass of a body, but resides 

 altogether at its surface. By the application of 

 mathematical calculations to the theory, the most 

 exact information with regard to the distribution of 

 the electric fluid in bodies of different shapes has 

 been obtained ; and whenever a comparison has been 

 instituted, even in the cases of the most complicated 

 kind, between the results of experiment and ot theory, 

 the most perfect agreement has been observed 

 between them. For the purpose of measuring the 

 proportional quantities of electricity, with which dif- 

 ferent parts of the same or of different bodies are 

 charged, no instrument is so well fitted as the balance 

 of Coulomb. Such is its extreme sensibility, that a 

 force only equal to the 270th of a grain is sufficient 

 to make the needle perform an entire revolution ; the 

 360th part of this force, therefore, or less than the 

 100,000th of a grain, is capable of being estimated 

 by each degree of its angular motion. It would be 

 inconsistent with the limits of the present article to 

 go into a detail of the delicate methods of research 

 adopted in the investigation of this subject. The 

 following are among some of the most interesting 

 results deduced from them. In a solid body, having 

 the form of a perfect sphere, and charged with posi- 

 tive electricity, the whole of the fluid is, in conse- 

 quence of the -repulsion of its own particles, which is 

 everywhere directed from the centre outwards, accu- 

 mulated in a thin stratum, at the very surface of the 

 sphere. If the body be charged with negative elec- 

 tricity, the deficiency of fluid will take place only in 

 the superficial stratum of matter. If, instead o 

 being spherical, the body have any other form, the 

 electricity will be chiefly confined to the surface , 

 and if it have an elongated form, there will be a 

 greater charge in the remoter parts than in those 

 nearer to the middle. This result of theory, respect- 

 ing the limitation of electricity to the mere surface 

 is confirmed, in the most decisive manner, by th< 

 experiments of Coulomb. A conducting body, of a 

 spheroidal shape, with small pits in various parts o 

 its surface, half an inch in diameter, and one-tent! 

 of an inch in depth, was electrified, and examinee 

 by the torsion balance. The bottoms of these pit 

 afforded no indications of having received any elec 

 tricity, while the even surface exhibited strong elec 

 trical excitement. We may conclude, both fron 

 theory and experiment, therefore, that although 

 strictly speaking, the electricity must reside within 

 the. substance of conducting bodies, it extends, L 

 fact, to a depth so small as to be inappreciable b 

 wiy known methods of observation. The effect o 



n expansion of surface, In lessening the intensity of 

 lectricity, while its absolute quantity remains the 

 ;imr. is illustrated by the following experiment. 

 Around an insulated cylinder, movable on a horizon- 

 tal axis, and turned by an insulating handle, is wound 

 A thin lamina of any metal, the end of which is semi- 

 ircular, and lias attached to it a silk thread. The 

 tvhole apparatus communicates with an electrometer, 

 ormed of two linen threads, each terminating in a 

 itli ball. On communicating a charge of electricity 

 to the cylinder, the threads and balls of the electro- 

 meter attached to it, diverge. Upon taking hold of 

 he silk thread, and unrolling the metallic lamina from 

 he cylinder, the balls gradually collapse, thus indi- 

 cating a diminution in the intensity of electrical 

 repulsion. But, on winding up theJamina, by turn- 

 ng the insulating handle, the electricity is restored, 

 and the balls diverge to the same extent as before, 

 allowance being made for the small dissipation of 

 lectricity, from the contact of the air during the 

 xperiment. In the case of a long and slender lamina 

 of conducting matter, charged with electricity, Cou- 

 omb found that its intensity continued nearly uniform, 

 rom the middle of the lamina to within a short dis- 

 tance from the ends ; at tliat part it rapidly increased; 

 and at the very extremity, it became twice as much 

 as at the middle part. He also found, that in a 

 cylinder 30 inches long and 2 in diameter, the inten- 

 sity of the electricity at the ends was to its intensity 

 n the middle, or at any part more than 2 inches from 

 the extremity, as 2.3 to 1. From which instances 

 we infer, that if a conducting substance be drawn out 

 into a point, the intensity of the electricity at that 

 point will be exceedingly great ; and that the point 

 will, accordingly, absorb and draw into itself nearly 

 the whole of the electricity that is contained in the 

 body. This great concentration of electricity is 

 found actually to take place in all points that project 

 beyond the general surface. The pressure excited 

 by the electric fluid against a non-conducting medium, 

 such as the air, which opposes an obstacle to its 

 escape, is in a ratio compounded of the repulsive force 

 of its own particles at the surface of the stratum of 

 fluid, and of the thickness of that stratum ; but as 

 one of these elements is always proportional to the 

 other, the total pressure must, in every point, be 

 proportional to the square of the thickness. If this 

 pressure be less than the resistance, or coercive force, 

 as it has been called, of the air, the electricity is 

 retained ; but the moment it exceeds that force, in 

 any one point, the electricity suddenly escapes, just 

 as a fluid confined in a vessel would rush out, if it 

 were to burst open a hole in the side of the vessel. 

 The eruption of the electric fluid is marked by seve- 

 ral very striking phenomena. A sharp snap is heard, 

 accompanied by a vivid spark, and there are eviden- 

 ces of an intense heat being evolved in the line which 

 the electricity takes. Its passage through a perfect 

 conductor is unattended with light. Light appears 

 only where there are obstacles hi its path, by the 

 interposition of imperfect conductors ; and such is 

 the velocity with which it is transmitted, that the 

 sparks appear to take place at the very same instant, 

 along the whole line of its course. Thus, if a row of 

 small fragments of tin-foil be pasted so as to be nearly 

 in contact, on a piece of glass, and electricity be sent 

 through them, by connecting one of its ends with the 

 conductor of an electrical machine, while the other 

 end communicates with the ground, it will not be 

 possible to detect any difference of time in the occur- 

 rence of the light in the different parts. If the tin- 

 foil be arranged so as to represent a chain, it will 

 appear luminous at each link, while conveying a 

 charge of electricity. Let A B, Fig. 14., represent 

 a glass tube, about a foot in length, and an inch in 



